The UK says it has concluded that the Russian A321 jet that crashed over Egypt’s Sinai on Saturday was likely brought down by an “explosive device.” The British government decided at a crisis meeting to ground all UK passenger flights to and from Sharm el-Sheikh.

“We have concluded that there is a significant possibility that the crash was caused by an explosive device on board the aircraft… We are now advising against all but essential travel by air through Sharm el-Sheikh airport. That means that there will be no UK passenger flights out to Sharm el-Sheikh from now,” Philip Hammond, the UK Secretary of State for Foreign and Commonwealth Affairs, told reporters after a meeting of a crisis response committee chaired by Prime Minister David Cameron.

An updated statement from Cameron’s office said that all flights to and from Sharm el-Sheikh have been suspended “with immediate effect.”

“We have consular staff at the airport who are working with the operators to ensure that all those passengers who were due to leave this evening are being looked after and taken to hotels,” the statement assured. It added that the government will “work urgently with the airlines and the Egyptian authorities with the aim of getting some flights up and running as soon as possible so that we can get people already in Sharm el-Sheikh… back safely to the UK as soon as possible.”

There will be no flights returning from Sharm el-Sheikh tomorrow, as the measures will “take time,” it warned.

Earlier, all flights from Egypt’s Sharm el-Sheikh resort to the UK had been delayed as a "precautionary measure," the joint statement from the Prime Minister's Office, the Department for Transport and Foreign & Commonwealth Office said.

A team of UK aviation experts was ordered to assess the security situation in the resort from where the Russian-operated Airbus 321 departed Saturday.

The Irish Aviation Authority has directed the country’s airlines to suspend operations to and from Sharm el-Sheikh Airport as well. The world’s largest charter airline, Thomson Airways, which flies from the UK and Ireland, has also temporarily suspended its flights to and from the Egyptian resort destination with immediate effect.

"While the investigation is still ongoing we cannot say categorically why the Russian jet crashed. But as more information has come to light we have become concerned that the plane may well have been brought down by an explosive device," the UK statement read.

The security assessment is expected to be completed by Wednesday night.

Extra consular staff will be deployed to the Egyptian resort, UK officials said, adding that they will be “on hand at the airport, working with the airlines, to assist British holidaymakers there.”

Since the Airbus A321 belonging to the Russian Metrojet air carrier crashed in Egypt, British officials “have been following the investigation closely... to ensure the safety of British citizens on flights from Sharm,” the joint statement said.

Earlier on Wednesday, Downing Street announced that British PM David Cameron and Egyptian President Abdel Fattah el-Sisi had discussed the Russian plane crash in Sinai in a phone conversation. The two officials “agreed it was important not to prejudge the investigation,” but with the cause of the crash still uncertain, they decided “it would be prudent to ensure the tightest possible security arrangements at Sharm el-Sheikh airport.”

US intelligence has suggested a bomb planted on the Russian passenger plane by Islamic State or an affiliate group is “most likely” behind the Metrojet flight crash, CNN reported on Wednesday, citing a US official familiar with the matter. The US intelligence community has not reached a formal conclusion, the source added, but said “there is a definite feeling it was an explosive device planted in luggage or somewhere on the plane.”

The Egyptian leader had said earlier that speculation that Islamic State might be behind the Russian plane crash was “false propaganda” aimed at damaging Egypt’s image.

A source close to the Egyptian investigation decoding the black boxes said that an explosion could possibly have caused the crash, Reuters reported, adding that it was unclear whether such a blast would have been the result of a bomb or fuel explosion.

Citing sources in Egypt’s investigative committee, Egypt’s Al-Masry Al-Youm newspaper reported on Wednesday that the decoded black boxes showed that an engine blast had caused the plane to crash, killing all 224 people on board.

With no distress signal sent from the plane to the flight’s control center, the anonymous source told the media that the explosion had been huge and could have affected all the engines at once. “The investigation did not point yet to have any links to terrorists,” Al-Masry Al-Youm cited its source as saying, adding that samples from the wreckage and the bodies had been taken to determine whether any explosive materials were present on the plane, or if the blast was the result of a mechanical failure.

Security has been beefed up at Sharm el-Sheikh airport, the Telegraph reported, adding that policemen with bulletproof vests were checking cars entering from outside the hub on Wednesday.

On the day of the crash, the Islamic State (IS, formerly ISIS/ISIL) terrorist group claimed responsibility for bringing down the plane. Another Islamic State video released on Tuesday showed a Slavic-looking and Russian-speaking jihadist praising his “Sinai brothers” in Egypt’s Sinai Peninsula for “taking down” the Russian jet and threatening more attacks in retaliation for Russia’s air campaign against IS in Syria.

The terrorists’ claims have been dubbed by Moscow and Cairo as “unlikely,” with officials saying the terrorist group does not possess the means to shoot down a plane at the altitude at which the Airbus was flying.

Although there has been no official announcement on the results of forensic medical examinations of the crash victims so far, there have been conflicting anonymous reports on the condition of the bodies. Russian tabloid LifeNews claimed to have obtained the results of a forensic medical examination that allegedly stated that the passengers “in the tail section of the liner died because of so-called blast injuries.” Burns covered over 90 percent of the victims’ bodies, which were pierced by particles of metal and aircraft covering, according to the report cited by the broadcaster.

On Tuesday, an Egyptian doctor who had examined the victims’ bodies suggested that “a powerful explosion took place aboard the plane before it hit the ground.” The nature of the injuries led him to make such a claim, the doctor told Sputnik news agency.

However, TASS news agency cited Russian and Egyptian experts as saying that they had failed to find any blast-related trauma during their preliminary examination of the victims’ bodies.

"There were no signs of an explosion impact found during the preliminary examination,” a Russian source said, with another Egyptian expert adding that “there were no signs of external impact” found on the bodies.

Some major international air carriers have avoided flying over the Sinai Peninsula area since Saturday’s crash. The biggest airline in the region, Emirates, as well as European Lufthansa and Air France-KLM have rerouted their flights until all the risks, including a possible terrorist act, have been ruled out by investigators.